The Computer End

Here's the Arduino interface. Black and yellow wires at left come from the reed switch on the bike. Pin 2 senses the reed switch (yellow wire). The 100ohm resistor is to protect the Arduino in case pin 2 gets configures as an output by mistake. A 10k pullup resistor connects to the 5v pin (red wire). Then LED is activated by pin 12 (green wire)  - it is programed to change stage as the magnet passes the reed switch and is helpful for quickly checking the system works.